knocking from the central heating, why?

got a combi boiler, 10 years old, every now and again it bangs, 2-4 times, every 20 mins or so. i've noticed the pressure gauge fluctuates when this happens, what is it? an air lock? i've tried bleeding rads and pump, but it still happens... help

Well that has that then. It's the diverter shutting off after pre heat/water usage.

Check you have no dripping hot taps or leaks on the hot water side of things, as cooling of the plate hear exchanger due to heat loss, will cause the boiler to go into Domestic Hot Water preheat more often than it should.